In previous jobs I have been HR and an accounting manager. The following is based on having been the interviewer, as well as interviewee.
The first thirty seconds is critical. Dress appropriately for the type of job in your area. Being slightly overdressed never hurts. Minimal jewelry, perfume/cologne. Do use a good deoderant, toothbrush, toothpaste, mouthwash and a breath mint just before you go through the front door. No I'm not kidding. This is especially true if you smoke.
Take an extra copy of your resume, and a cheat sheet with the names, addresses phone numbers etc for previous employers. This is for your use when filling out their application.
Make sure you know where your going. Leave early in case traffic is bad. If you get there too early (more than about ten minutes before the appointment time, walk around the block and relax, deep breathing helps.
Take your cell phone (but turn it off before you go into the interview). Make sure you have a name and phone number with you. Being late is a killer, calling on your cell phone and saying "the freeway is a parking lot and the nearest exit is a mile away. I can be there in half an hour or we can reschedule" can actually make you look good, assuming there was in fact an accident or something on the freeway.
LOOK THE INTERVIEW IN THE EYE, SMILE, BRIEF BUT FIRM HANDSHAKE.